1. Field of the Invention
The invention relates to a process for the production of catalytic converters for purifying exhaust gases, and more particularly to a method for producing the catalytic converter comprising pre-forming the metal shell and compressing the supporting mat material against the metal shell.
2. Description of the Related Art
As is well known, the purification of exhaust gases from internal combustion engines, particularly in motor vehicles, is generally achieved by an exhaust gas purification system in which a ceramic element having a honeycomb cell structure acts a catalyst carrier. More precisely, this honeycomb cell structure is covered with a catalyst that contains a precious metal which functions, in the presence of O2, to convert noxious components of the exhaust gas, such as HC and CO, to H2O and CO2. The honeycomb cell structure is housed within a gas-tight, sheet metal or cast-metal heat resistant housing or can/shell.
Honeycomb structures currently employed are typically comprised of a ceramic material such as cordierite; a brittle material exhibiting limited mechanical strength. For this reason, catalytic converters in use today, typically include a resilient supporting mat that is wrapped around the periphery of the honeycomb. This resilient material, which distributes any compressive forces uniformly on the ceramic, typically expands as the temperature increases. This being the case, the compressive supporting pressure on the honeycomb therefore increases at elevated temperatures, and in some degree compensates for the thermal expansion of the outer metal shell. Since the metal shell expands more than the enclosed ceramic honeycomb, this mat expansion with temperature rise, prevents the honeycomb from becoming loose in the shell.
There are known in the art various techniques for assembling the mat and ceramic monolith into a can to make catalytic converters as described above. In general, the existing techniques can be divided into two groups: (1) those processes where preliminarily established gap, between the ceramic monolith and the metal shell is maintained during assembly (e.g., Stuff mounting or Clam Shell techniques); (2) those processes where a predetermined pressure, between the metal shell and the ceramic monolith is maintained during assembly (e.g., Shoe-box and Tourniquet techniques).
Stuff mounting techniques involve initially wrapping the substrate in a resilient mat and thereafter inserting the wrapped substrate into a conical device that compresses the mat as it is pushed through. The wrapped substrate is then ejected from the compression cone into a cylindrical tube that serves as the converter container or shell (see, for example U.S. Pat. No. 4,093,423 (Neumann).
Clam shell style of canning involves the utilization of two metal shell halves which are closed around a mat-wrapped honeycomb and thereafter welded together; (see for example U.S. Pat. No. 5,273,724 (Bos).
The method of fabrication, commonly referred to as the xe2x80x9ctourniquet wrapxe2x80x9d method, involves forming a rectangular flat sheet metal piece into a cylindrical body having a lap joint. A mat-wrapped honeycomb is loosely inserted into the cylindrical metal can and the combined assembly is pulled together to form the desired mat compression. Thereafter, the lap joint is welded together thereby holding the can at the desired compression while at the same time preventing gas leakage; see for Example U.S. Pat. No. 5,082,479 (Miller).
It is known that the amount of compressive pressure exerted on a given honeycomb substrate as a result of compressively closing the metal shell and supporting mat around the honeycomb substrate, as is done in any of the prior art methods described above, is significantly affected by the honeycomb""s outside diameter, the thickness and compliance of the supporting mat material and the metal shell dimensions. Each of these dimensions have manufacturing tolerances which must be carefully controlled to insure that adequate, but not excessive, radial pressure, is applied to the honeycomb substrate. The prior art methods of assembling catalytic converters discussed above all involve subjecting the ceramic substrate to uneven and indirect compressive forces during the assembly, as a result of the exertion of compression on the metal shell or compression of the aforementioned conical device. The compression on the metal shell can result in damage to the substrate from the crushing forces applied, with the risk of damage increasing in the case of advanced substrates having extremely thin cell walls and surrounding skin.
Other disadvantages of these prior art techniques include resultant gap variations, instantaneous pressure peaks due to high closure rate, non-uniform pressure distribution, especially with non-round monolith. Because the mat is a viscoelastic material at room temperature, the compression pressure is rate dependent, e.g., the faster it is compressed, the higher the pressure and thus the higher the resultant undesirable pressure peaks that the ceramic monolith is subjected to. Again this disadvantage becomes more problematic as the substrates produced and utilized exhibit thinner cell walls.
As such, there remains a need for, and it is thus an objective of this invention to provide, for a simpler, less labor-intensive, more efficient catalytic converter assembly process that achieves both a uniform mat density and a uniform compression on the ceramic substrate; particularly in a manner such that the maximum compression exerted at any time on the ceramic substrate does cause damage to the substrate. In particular, it is an objective of the instant invention, to disclose a method that avoids subjecting the ceramic monolith to undesirable pressure peaks that lead to high stresses in the brittle ceramic monoliths.
It is therefore an objective of the present invention to disclose an assembly method that overcomes the problems and shortcomings of the current compressive closing methods for assembling catalytic converters. In other words, the present invention discloses a method of assembling catalytic converters which achieves a compressive load upon the honeycomb structure which is sufficient to retain, but not damage the retained honeycomb substrate, and which is not subject to pressure peaks experienced by the prior art assembly methods.
This objective, as well as other objectives which will become apparent in the discussion that follows, are achieved, in accordance with the present invention as a result of the feature that the initial compression of the supporting mat is a result of the mat being compressed against metal shell; i.e., internal compression of the xe2x80x9cmat against the shellxe2x80x9d. This is contrary to the compression in the prior art methods that involve the initial compression being external of the supporting mat and resulting in the metal shell compressing the supporting mat against the ceramic substrate, i.e., xe2x80x9cmetal shell and/or mat against the substratexe2x80x9d. In other words, the instant assembly method comprises providing an encircled mat layered metal shell and compressing of the mat layer against the metal shell and subsequently releasing the compression of the supporting mat thereby subjecting the ceramic substrate to the far less intensive release compression of the supporting mat.
In general, the method of assembling these catalytic converters comprises the following steps: (1) providing an open-ended one piece metal shell; (2) positioning a layer of resilient mat material on the inside surface of the metal shell to form an encircling mat layer; (3) compressing the encircling mat layer; and, (4) inserting the ceramic substrate into the metal shell while retaining the encircling mat layer on the inside surface of the metal shell.
In a more detailed embodiment the method of assembling these catalytic converters comprises the following basic steps: (1) providing a metal shell exhibiting a predetermined shape that substantially matches the shape of the ceramic substrate; (2) inserting into the metal shell a sufficient amount of a resilient supporting mat material to form an encircling mat layer; (3) compressing the encircling mat layer to an initial gap bulk density, the initial gap bulk density being equal to or higher than a predetermined final gap bulk density; (4) releasing the compression on the mat layer and, prior to the mat layer reaching its final gap bulk density, inserting at least a portion of the substrate into the encircling mat layered metal shell and then allowing the mat layer to further release until the mat layer is compressed against the ceramic substrate at the final predetermined gap bulk density.